1. Field of the Invention
The present invention relates to the art of railway cab signalling systems. More particularly, the invention relates to a system and a method of utilizing typical coded track circuit signals to provide cab signalling information.
2. Description of the Prior Art
Movement of a railway vehicle along a railroad is necessarily limited to one degree of freedom. That is to say, the vehicle can only travel back and forth along the track. It cannot alter its course to avoid other traffic. To prevent railway vehicles on the same track from overtaking each other, a block signalling scheme has been devised whereby the track is divided into segments, or "blocks," of a length greater than the stopping distance of a train. To prevent a problem, only one train is allowed in a particular block at a time. Wayside block indicators positioned before an upcoming block indicate to the locomotive engineer whether or not the block is occupied. If so, the engineer will know to adjust the speed of the train.
The operation of wayside block indicators has been traditionally controlled by the track circuit. The track circuit is essentially an electrical circuit in which the rails in a block complete a connection between an electrical signal transmitter and an electrical signal receiver. Insulating joints may be placed between adjacent blocks to provide electrical separation. When the block is unoccupied, current is allowed to flow through the rails to the receiver. The receiver, such as a relay, can then activate the wayside indicator to display an appropriate aspect. If, however, the block is occupied by any part of a train, shunt paths are created by the presence of wheel and axle sets on the train. Typically, most current is shunted through the wheel and axle set closest to the signal transmitter. Since the current is prevented from reaching the receiver, the wayside indicator will typically give a stop signal or simply no signal at all.
Originally, track circuits utilized only direct current. The block length was limited in these systems due to electrical leakage through the ballast between the rails and foreign ground currents which could enter the system. It was subsequently found that a pulse modulated current would facilitate the use of a more sensitive relay. This increased the operable track circuit length in main-line areas to 15,000 feet or more. It also allowed the track circuit current to carry coded information which could be utilized by the wayside indicators to provide additional signal aspects.
While wayside indicators are generally effective in providing information to the locomotive engineer, their usefulness may be reduced during periods of fog or other inclement weather. Thus, in order to supplement the wayside indicators, cab signalling was developed. Using traffic control indicators located on-board the vehicle, cab signalling provides locomotive engineers with continuous signalling information similar to that provided by wayside indicators.
Present cab signalling systems typically operate using a receiver on a locomotive inductively coupled to the track. Specifically, a pick-up coil is mounted on a supporting structure depending from the locomotive such that the coil is ahead of the leading axle and approximately six inches above the rail. The coil senses the presence of a modulated AC carrier. While sometimes coded to repeat the governing wayside aspect, the frequency of the cab signalling carrier is generally higher than the coded track circuit signal in order to provide effective inductive coupling to the pick-up coil. Thus, a block signalling system having both wayside indicators and cab signalling will generally have two superimposed electrical signals in the track: the coded track circuit signal and the modulated carrier cab signalling signal.
The carrier signal has been a deterrent to more prevalent utilization of cab signalling. This is due, in part, to the distance limitation imposed by the carrier. For example, a cab signalling system having a typical carrier frequency of 100 hertz will have a range of only about 6,000 feet. This may add cost to the overall signalling system since additional wayside equipment is required. Additional insulating joints may also be necessary, further adding cost to the overall system.
In the early 1950s, attempts were made to improve cab signalling systems by eliminating the carrier and detecting the coded track circuit current using magnetic field sensors mounted above the rails. Without the carrier, the track circuit length could be increased to its maximum and system costs could be reduced. The attempts to develop such a system, however, were a failure. This failure has been attributed to interference caused by magnetized tie plates. Since the sensors were mounted above the rails, they sensed the combination of the field from the rail current as well as the effects produced by the tie plates.